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Abstract:

A split-cycle air-hybrid engine includes a rotatable crankshaft. A
compression piston is slidably received within a compression cylinder and
operatively connected to the crankshaft. An intake valve selectively
controls air flow into the compression cylinder. An expansion piston is
slidably received within an expansion cylinder and operatively connected
to the crankshaft. A crossover passage interconnects the compression and
expansion cylinders. The crossover passage includes a crossover
compression (XovrC) valve and crossover expansion (XovrE) valve therein.
An air reservoir is operatively connected to the crossover passage. An
air reservoir valve selectively controls air flow into and out of the air
reservoir. In a Firing and Charging (FC) mode of the engine, the air
reservoir valve is kept closed until the XovrE valve is substantially
closed during a single rotation of the crankshaft such that the expansion
cylinder is charged with compressed air before the air reservoir is
charged with compressed air.

Claims:

1. A split-cycle air-hybrid engine comprising: a crankshaft rotatable
about a crankshaft axis; a compression piston slidably received within a
compression cylinder and operatively connected to the crankshaft such
that the compression piston reciprocates through an intake stroke and a
compression stroke during a single rotation of the crankshaft; an intake
valve selectively controlling air flow into the compression cylinder; an
expansion piston slidably received within an expansion cylinder and
operatively connected to the crankshaft such that the expansion piston
reciprocates through an expansion stroke and an exhaust stroke during a
single rotation of the crankshaft; a crossover passage interconnecting
the compression and expansion cylinders, the crossover passage including
a crossover compression (XovrC) valve and a crossover expansion (XovrE)
valve defining a pressure chamber therebetween; an air reservoir
operatively connected to the crossover passage and selectively operable
to store compressed air from the compression cylinder; and an air
reservoir valve selectively controlling air flow into and out of the air
reservoir; the engine being operable in a Firing and Charging (FC) mode,
wherein, in the FC mode, the air reservoir valve is kept closed until the
XovrE valve is substantially closed during a single rotation of the
crankshaft such that the expansion cylinder is charged with compressed
air before the air reservoir is charged with compressed air.

2. The split-cycle air-hybrid engine of claim 1, wherein, in the FC mode,
the air reservoir valve remains closed in a range of from within plus or
minus 5 degrees CA of when the XovrC valve opens to within plus or minus
5 degrees CA of when the XovrE valve closes.

3. The split-cycle air-hybrid engine of claim 1, wherein, in the FC mode,
the air reservoir valve opens at a position 5 degrees CA or greater after
the XovrE valve closes.

4. The split-cycle air-hybrid engine of claim 1, wherein, in the FC mode,
the air reservoir valve opens at a position in a range of 5-20 degrees CA
after the XovrE valve closes.

5. The split-cycle air-hybrid engine of claim 1, wherein, in the FC mode,
the air reservoir valve opens at a position less than 10 degrees CA after
the XovrE valve closes.

6. The split-cycle air-hybrid engine of claim 1, wherein, in the FC mode,
the air reservoir valve is kept open for a duration of 25 degrees CA or
greater.

7. The split-cycle air-hybrid engine of claim 1, wherein, in the FC mode,
the air reservoir valve is kept open for a duration of 50 degrees CA or
greater.

8. The split-cycle air-hybrid engine of claim 1, wherein, in the FC mode,
the air reservoir valve is kept open for a duration within a range of 25
degrees CA to 150 degrees CA.

9. The split-cycle air-hybrid engine of claim 1, wherein, in the FC mode,
engine load is controlled by controlling the timing of XovrE valve
closing.

10. The split-cycle air-hybrid engine of claim 1, wherein, in the FC
mode, an amount of excess compressed air delivered to the air reservoir
is controlled by controlling the timing of intake valve closing.

11. The split-cycle air-hybrid engine of claim 1, wherein, in the FC
mode, the compression piston draws in and compresses inlet air for use in
the expansion cylinder, and compressed air is admitted to the expansion
cylinder with fuel, at the beginning of an expansion stroke, which is
ignited, burned and expanded on the same expansion stroke of the
expansion piston, transmitting power to the crankshaft, and the
combustion products are discharged on the exhaust stroke.

12. A method of operating a split-cycle air-hybrid engine including: a
crankshaft rotatable about a crankshaft axis; a compression piston
slidably received within a compression cylinder and operatively connected
to the crankshaft such that the compression piston reciprocates through
an intake stroke and a compression stroke during a single rotation of the
crankshaft; an intake valve selectively controlling air flow into the
compression cylinder; an expansion piston slidably received within an
expansion cylinder and operatively connected to the crankshaft such that
the expansion piston reciprocates through an expansion stroke and an
exhaust stroke during a single rotation of the crankshaft; a crossover
passage interconnecting the compression and expansion cylinders, the
crossover passage including a crossover compression (XovrC) valve and a
crossover expansion (XovrE) valve defining a pressure chamber
therebetween; an air reservoir operatively connected to the crossover
passage and selectively operable to store compressed air from the
compression cylinder; and an air reservoir valve selectively controlling
air flow into and out of the air reservoir; the engine being operable in
a Firing and Charging (FC) mode; the method including the steps of:
drawing in and compressing inlet air with the compression piston;
admitting compressed air from the compression cylinder into the expansion
cylinder with fuel, at the beginning of an expansion stroke, the fuel
being ignited, burned and expanded on the same expansion stroke of the
expansion piston, transmitting power to the crankshaft, and the
combustion products being discharged on the exhaust stroke; and keeping
the air reservoir valve closed until the XovrE valve is substantially
closed during a single rotation of the crankshaft such that the expansion
cylinder is charged with compressed air before the air reservoir is
charged with compressed air.

13. The method of claim 12, including the step of keeping the air
reservoir valve closed in a range of from within plus or minus 5 degrees
CA of when the XovrC valve opens to within plus or minus 5 degrees CA of
when the XovrE valve closes.

14. The method of claim 12, including the step of opening the air
reservoir valve at a position 5 degrees CA or greater after the XovrE
valve closes.

15. The method of claim 12, including the step of opening the air
reservoir valve at a position in a range of 5-20 degrees CA after the
XovrE valve closes.

16. The method of claim 12, including the step of opening the air
reservoir valve at a position less than 10 degrees CA after the XovrE
valve closes.

17. The method of claim 12, including the step of keeping the air
reservoir valve open for a duration of 25 degrees CA or greater.

18. The method of claim 12, further including the step of controlling
engine load by varying the timing of XovrE valve closing.

19. The method of claim 12, further including the step of controlling an
amount of excess compressed air delivered to the air reservoir by varying
the timing of intake valve closing.

[0002] This invention relates to split-cycle engines and, more
particularly, to such an engine incorporating an air-hybrid system.

BACKGROUND OF THE INVENTION

[0003] For purposes of clarity, the term "conventional engine" as used in
the present application refers to an internal combustion engine wherein
all four strokes of the well-known Otto cycle (i.e., the intake (or
inlet), compression, expansion (or power) and exhaust strokes) are
contained in each piston/cylinder combination of the engine. Each stroke
requires one half revolution of the crankshaft (180 degrees crank angle
(CA)), and two full revolutions of the crankshaft (720 degrees CA) are
required to complete the entire Otto cycle in each cylinder of a
conventional engine.

[0004] Also, for purposes of clarity, the following definition is offered
for the term "split-cycle engine" as may be applied to engines disclosed
in the prior art and as referred to in the present application.

[0005] A split-cycle engine as referred to herein comprises:

[0006] a crankshaft rotatable about a crankshaft axis;

[0007] a compression piston slidably received within a compression
cylinder and operatively connected to the crankshaft such that the
compression piston reciprocates through an intake stroke and a
compression stroke during a single rotation of the crankshaft;

[0008] an expansion (power) piston slidably received within an expansion
cylinder and operatively connected to the crankshaft such that the
expansion piston reciprocates through an expansion stroke and an exhaust
stroke during a single rotation of the crankshaft; and

[0009] a crossover passage (port) interconnecting the compression and
expansion cylinders, the crossover passage including at least a crossover
expansion (XovrE) valve disposed therein, but more preferably including a
crossover compression (XovrC) valve and a crossover expansion (XovrE)
valve defining a pressure chamber therebetween.

[0010] U.S. Pat. No. 6,543,225 granted Apr. 8, 2003 to Scuderi and U.S.
Pat. No. 6,952,923 granted Oct. 11, 2005 to Branyon et al., both of which
are incorporated herein by reference, contain an extensive discussion of
split-cycle and similar-type engines. In addition, these patents disclose
details of prior versions of an engine of which the present disclosure
details further developments.

[0011] Split-cycle air-hybrid engines combine a split-cycle engine with an
air reservoir and various controls. This combination enables a
split-cycle air-hybrid engine to store energy in the form of compressed
air in the air reservoir. The compressed air in the air reservoir is
later used in the expansion cylinder to power the crankshaft.

[0012] A split-cycle air-hybrid engine as referred to herein comprises:

[0013] a crankshaft rotatable about a crankshaft axis;

[0014] a compression piston slidably received within a compression
cylinder and operatively connected to the crankshaft such that the
compression piston reciprocates through an intake stroke and a
compression stroke during a single rotation of the crankshaft;

[0015] an expansion (power) piston slidably received within an expansion
cylinder and operatively connected to the crankshaft such that the
expansion piston reciprocates through an expansion stroke and an exhaust
stroke during a single rotation of the crankshaft;

[0016] a crossover passage (port) interconnecting the compression and
expansion cylinders, the crossover passage including at least a crossover
expansion (XovrE) valve disposed therein, but more preferably including a
crossover compression (XovrC) valve and a crossover expansion (XovrE)
valve defining a pressure chamber therebetween; and

[0017] an air reservoir operatively connected to the crossover passage and
selectively operable to store compressed air from the compression
cylinder and to deliver compressed air to the expansion cylinder.

[0018] U.S. Pat. No. 7,353,786 granted Apr. 8, 2008 to Scuderi et al.,
which is incorporated herein by reference, contains an extensive
discussion of split-cycle air-hybrid and similar-type engines. In
addition, this patent discloses details of prior hybrid systems of which
the present disclosure details further developments.

[0019] A split-cycle air-hybrid engine can be run in a normal operating or
firing (NF) mode (also commonly called the Engine Firing (EF) mode) and
four basic air-hybrid modes. In the EF mode, the engine functions as a
non-air hybrid split-cycle engine, operating without the use of its air
reservoir. In the EF mode, a tank valve operatively connecting the
crossover passage to the air reservoir remains closed to isolate the air
reservoir from the basic split-cycle engine.

[0020] The split-cycle air-hybrid engine operates with the use of its air
reservoir in four hybrid modes. The four hybrid modes are: [0021] 1)
Air Expander (AE) mode, which includes using compressed air energy from
the air reservoir without combustion; [0022] 2) Air Compressor (AC) mode,
which includes storing compressed air energy into the air reservoir
without combustion; [0023] 3) Air Expander and Firing (AEF) mode, which
includes using compressed air energy from the air reservoir with
combustion; and [0024] 4) Firing and Charging (FC) mode, which includes
storing compressed air energy into the air reservoir with combustion.
However, further optimization of these modes, EF, AE, AC, AEF and FC, is
desirable to enhance efficiency and reduce emissions.

SUMMARY OF THE INVENTION

[0025] The present invention provides a split-cycle air-hybrid engine in
which the use of the Firing and Charging (FC) mode is optimized for
potentially any vehicle in any drive cycle for improved efficiency.

[0026] More particularly, an exemplary embodiment of a split-cycle
air-hybrid engine in accordance with the present invention includes a
crankshaft rotatable about a crankshaft axis. A compression piston is
slidably received within a compression cylinder and operatively connected
to the crankshaft such that the compression piston reciprocates through
an intake stroke and a compression stroke during a single rotation of the
crankshaft. An intake (or inlet) valve selectively controls air flow into
the compression cylinder. An expansion piston is slidably received within
an expansion cylinder and operatively connected to the crankshaft such
that the expansion piston reciprocates through an expansion stroke and an
exhaust stroke during a single rotation of the crankshaft. A crossover
passage interconnects the compression and expansion cylinders. The
crossover passage includes a crossover compression (XovrC) valve and a
crossover expansion (XovrE) valve defining a pressure chamber
therebetween. An air reservoir is operatively connected to the crossover
passage and selectively operable to store compressed air from the
compression cylinder. An air reservoir valve selectively controls air
flow into and out of the air reservoir. The engine is operable in a
Firing and Charging (FC) mode. In the FC mode, the air reservoir valve is
kept closed until the XovrE valve is substantially closed during a single
rotation of the crankshaft such that the expansion cylinder is charged
with compressed air before the air reservoir is charged with compressed
air.

[0027] A method of operating a split-cycle air-hybrid engine is also
disclosed. The split-cycle air-hybrid engine includes a crankshaft
rotatable about a crankshaft axis. A compression piston is slidably
received within a compression cylinder and operatively connected to the
crankshaft such that the compression piston reciprocates through an
intake stroke and a compression stroke during a single rotation of the
crankshaft. An intake valve selectively controls air flow into the
compression cylinder. An expansion piston is slidably received within an
expansion cylinder and operatively connected to the crankshaft such that
the expansion piston reciprocates through an expansion stroke and an
exhaust stroke during a single rotation of the crankshaft. A crossover
passage interconnects the compression and expansion cylinders. The
crossover passage includes a crossover compression (XovrC) valve and a
crossover expansion (XovrE) valve defining a pressure chamber
therebetween. An air reservoir is operatively connected to the crossover
passage and selectively operable to store compressed air from the
compression cylinder. An air reservoir valve selectively controls air
flow into and out of the air reservoir. The engine is operable in a
Firing and Charging (FC) mode. The method in accordance with the present
invention includes the following steps: drawing in and compressing inlet
(or intake) air with the compression piston; admitting compressed air
from the compression cylinder into the expansion cylinder with fuel, at
the beginning of an expansion stroke, the fuel being ignited, burned and
expanded on the same expansion stroke of the expansion piston,
transmitting power to the crankshaft, and the combustion products being
discharged on the exhaust stroke; and keeping the air reservoir valve
closed until the XovrE valve is substantially closed during a single
rotation of the crankshaft such that the expansion cylinder is charged
with compressed air before the air reservoir is charged with compressed
air.

[0028] These and other features and advantages of the invention will be
more fully understood from the following detailed description of the
invention taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In the drawings:

[0030]FIG. 1 is a lateral sectional view of an exemplary split-cycle
air-hybrid engine in accordance with the present invention;

[0031]FIG. 2 is a graphical illustration of intake (inlet) valve closing
timing with respect to tank air pressure and tank air flowrate at an
engine speed of 2000 revolutions per minute (rpm) and engine load of 2
bar Indicated Mean Effective Pressure (IMEP);

[0032]FIG. 3 is a graphical illustration of intake valve duration with
respect to tank air pressure and tank air flowrate at an engine speed of
2000 rpm and engine load of 2 bar IMEP;

[0033]FIG. 4 is a graphical illustration of crossover compression (XovrC)
valve duration with respect to tank air pressure and tank air flowrate at
an engine speed of 2000 rpm and engine load of 2 bar IMEP;

[0034]FIG. 5 is a graphical illustration of crossover expansion (XovrE)
valve duration with respect to tank air pressure and tank air flowrate at
an engine speed of 2000 rpm and engine load of 2 bar IMEP;

[0035]FIG. 6 is a graphical illustration of XovrC valve opening timing
with respect to tank air pressure and tank air flowrate at an engine
speed of 2000 rpm and engine load of 2 bar IMEP;

[0036]FIG. 7 is a graphical illustration of XovrC valve closing timing
with respect to tank air pressure and tank air flowrate at an engine
speed of 2000 rpm and engine load of 2 bar IMEP;

[0037]FIG. 8 is a graphical illustration of XovrE valve opening timing
with respect to tank air pressure and tank air flowrate at an engine
speed of 2000 rpm and engine load of 2 bar IMEP;

[0038]FIG. 9 is a graphical illustration of XovrE valve closing timing
with respect to tank air pressure and tank air flowrate at an engine
speed of 2000 rpm and engine load of 2 bar IMEP;

[0039]FIG. 10 is a graphical illustration of air tank valve opening
timing with respect to tank air pressure and tank air flowrate at an
engine speed of 2000 rpm and engine load of 2 bar IMEP;

[0040]FIG. 11 is a graphical illustration of air tank valve closing
timing with respect to tank air pressure and tank air flowrate at an
engine speed of 2000 rpm and engine load of 2 bar IMEP; and

[0041]FIG. 12 is a graphical illustration of fuel flowrate with respect
to tank air pressure for various tank air flowrates at an engine speed of
2000 rpm and engine load of 2 bar IMEP.

DETAILED DESCRIPTION OF THE INVENTION

[0042] The following glossary of acronyms and definitions of terms used
herein is provided for reference.

In General

[0043] Unless otherwise specified, all valve opening and closing timings
are measured in crank angle degrees after top dead center of the
expansion piston (ATDCe).

Air tank (or air storage tank): Storage tank for compressed air. ATDCe:
After top dead center of the expansion piston. Bar: Unit of pressure, 1
bar=105 N/m2 BMEP: Brake mean effective pressure. The term
"Brake" refers to the output as delivered to the crankshaft (or output
shaft), after friction losses (FMEP) are accounted for. Brake Mean
Effective Pressure (BMEP) is the engine's brake torque output expressed
in terms of a mean effective pressure (MEP) value. BMEP is equal to the
brake torque divided by engine displacement. This is the performance
parameter taken after the losses due to friction. Accordingly,
BMEP=IMEP-friction. Friction, in this case is usually also expressed in
terms of an MEP value known as Frictional Mean Effective Pressure (or
FMEP). Compressor: The compression cylinder and its associated
compression piston of a split-cycle engine. Expander: The expansion
cylinder and its associated expansion piston of a split-cycle engine.

FMEP: Frictional Mean Effective Pressure.

[0045] g/s: Grams per second. IMEP: Indicated Mean Effective Pressure. The
term "Indicated" refers to the output as delivered to the top of the
piston, before friction losses (FMEP) are accounted for. Inlet (or
intake): Inlet valve. Also commonly referred to as an intake valve. Inlet
air (or intake air): Air drawn into the compression cylinder on an intake
(or inlet) stroke. Inlet valve (or intake valve): Valve controlling
intake of gas into the compression cylinder.

RPM: Revolutions Per Minute.

[0046] Tank valve: Valve connecting the Xovr passage with the compressed
air storage tank. Valve duration: The interval in crank degrees between
start of valve opening and end of valve closing. VVA: Variable valve
actuation. A mechanism or method operable to alter the shape or timing of
a valve's lift profile. Xovr (or Xover) valve, passage or port: The
crossover valves, passages, and/or ports which connect the compression
and expansion cylinders through which gas flows from compression to
expansion cylinder. XovrC (or XoverC) valves: Valves at the compressor
end of the Xovr passage. XovrE (or XoverE) valves: Valves at the expander
end of the crossover (Xovr) passage.

[0047] Referring to FIG. 1, an exemplary split-cycle air-hybrid engine is
shown generally by numeral 10. The split-cycle air-hybrid engine 10
replaces two adjacent cylinders of a conventional engine with a
combination of one compression cylinder 12 and one expansion cylinder 14.
A cylinder head 33 is typically disposed over an open end of the
expansion and compression cylinders 12, 14 to cover and seal the
cylinders.

[0048] The four strokes of the Otto cycle are "split" over the two
cylinders 12 and 14 such that the compression cylinder 12, together with
its associated compression piston 20, perform the intake (or inlet) and
compression strokes, and the expansion cylinder 14, together with its
associated expansion piston 30, perform the expansion (or power) and
exhaust strokes. The Otto cycle is therefore completed in these two
cylinders 12, 14 once per crankshaft 16 revolution (360 degrees CA) about
crankshaft axis 17.

[0049] During the intake stroke, intake (or inlet) air is drawn into the
compression cylinder 12 through an intake port 19 disposed in the
cylinder head 33. An inwardly opening (opening inwardly into the cylinder
and toward the piston) poppet intake (or inlet) valve 18 controls fluid
communication between the intake port 19 and the compression cylinder 12.

[0050] During the compression stroke, the compression piston 20
pressurizes the air charge and drives the air charge into the crossover
passage (or port) 22, which is typically disposed in the cylinder head
33. This means that the compression cylinder 12 and compression piston 20
are a source of high-pressure gas to the crossover passage 22, which acts
as the intake passage for the expansion cylinder 14. In some embodiments,
two or more crossover passages 22 interconnect the compression cylinder
12 and the expansion cylinder 14.

[0051] The geometric (or volumetric) compression ratio of the compression
cylinder 12 of split-cycle engine 10 (and for split-cycle engines in
general) is herein commonly referred to as the "compression ratio" of the
split-cycle engine. The geometric (or volumetric) compression ratio of
the expansion cylinder 14 of split-cycle engine 10 (and for split-cycle
engines in general) is herein commonly referred to as the "expansion
ratio" of the split-cycle engine. The geometric compression ratio of a
cylinder is well known in the art as the ratio of the enclosed (or
trapped) volume in the cylinder (including all recesses) when a piston
reciprocating therein is at its bottom dead center (BDC) position to the
enclosed volume (i.e., clearance volume) in the cylinder when said piston
is at its top dead center (TDC) position. Specifically for split-cycle
engines as defined herein, the compression ratio of a compression
cylinder is determined when the XovrC valve is closed. Also specifically
for split-cycle engines as defined herein, the expansion ratio of an
expansion cylinder is determined when the XovrE valve is closed.

[0052] Due to very high compression ratios (e.g., to 1, 30 to 1, 40 to 1,
or greater) within the compression cylinder 12, an outwardly opening
(opening outwardly away from the cylinder) poppet crossover compression
(XovrC) valve 24 at the crossover passage inlet 25 is used to control
flow from the compression cylinder 12 into the crossover passage 22. Due
to very high expansion ratios (e.g., 20 to 1, 30 to 1, 40 to 1, or
greater) within the expansion cylinder 14, an outwardly opening poppet
crossover expansion (XovrE) valve 26 at the outlet 27 of the crossover
passage 22 controls flow from the crossover passage 22 into the expansion
cylinder 14. The actuation rates and phasing of the XovrC and XovrE
valves 24, 26 are timed to maintain pressure in the crossover passage 22
at a high minimum pressure (typically 20 bar or higher at full load)
during all four strokes of the Otto cycle.

[0053] At least one fuel injector 28 injects fuel into the pressurized air
at the exit end of the crossover passage 22 in correspondence with the
XovrE valve 26 opening, which occurs shortly before expansion piston 30
reaches its top dead center position. The air/fuel charge enters the
expansion cylinder 14 when expansion piston 30 is close to its top dead
center position. As piston 30 begins its descent from its top dead center
position, and while the XovrE valve 26 is still open, spark plug 32,
which includes a spark plug tip 39 that protrudes into cylinder 14, is
fired to initiate combustion in the region around the spark plug tip 39.
Combustion can be initiated while the expansion piston is between 1 and
30 degrees CA past its top dead center (TDC) position. More preferably,
combustion can be initiated while the expansion piston is between 5 and
25 degrees CA past its top dead center (TDC) position. Most preferably,
combustion can be initiated while the expansion piston is between 10 and
20 degrees CA past its top dead center (TDC) position. Additionally,
combustion may be initiated through other ignition devices and/or
methods, such as with glow plugs, microwave ignition devices or through
compression ignition methods.

[0054] During the exhaust stroke, exhaust gases are pumped out of the
expansion cylinder 14 through exhaust port 35 disposed in cylinder head
33. An inwardly opening poppet exhaust valve 34, disposed in the inlet 31
of the exhaust port 35, controls fluid communication between the
expansion cylinder 14 and the exhaust port 35. The exhaust valve 34 and
the exhaust port 35 are separate from the crossover passage 22. That is,
exhaust valve 34 and the exhaust port 35 do not make contact with, or are
not disposed in, the crossover passage 22.

[0055] With the split-cycle engine concept, the geometric engine
parameters (i.e., bore, stroke, connecting rod length, volumetric
compression ratio, etc.) of the compression 12 and expansion 14 cylinders
are generally independent from one another. For example, the crank throws
36, 38 for the compression cylinder 12 and expansion cylinder 14,
respectively, may have different radii and may be phased apart from one
another such that top dead center (TDC) of the expansion piston 30 occurs
prior to TDC of the compression piston 20. This independence enables the
split-cycle engine 10 to potentially achieve higher efficiency levels and
greater torques than typical four-stroke engines.

[0056] The geometric independence of engine parameters in the split-cycle
engine 10 is also one of the main reasons why pressure can be maintained
in the crossover passage 22 as discussed earlier. Specifically, the
expansion piston 30 reaches its top dead center position prior to the
compression piston reaching its top dead center position by a discreet
phase angle (typically between 10 and 30 crank angle degrees). This phase
angle, together with proper timing of the XovrC valve 24 and the XovrE
valve 26, enables the split-cycle engine 10 to maintain pressure in the
crossover passage 22 at a high minimum pressure (typically 20 bar
absolute or higher during full load operation) during all four strokes of
its pressure/volume cycle. That is, the split-cycle engine 10 is operable
to time the XovrC valve 24 and the XovrE valve 26 such that the XovrC and
XovrE valves are both open for a substantial period of time (or period of
crankshaft rotation) during which the expansion piston 30 descends from
its TDC position towards its BDC position and the compression piston 20
simultaneously ascends from its BDC position towards its TDC position.
During the period of time (or crankshaft rotation) that the crossover
valves 24, 26 are both open, a substantially equal mass of air is
transferred (1) from the compression cylinder 12 into the crossover
passage 22 and (2) from the crossover passage 22 to the expansion
cylinder 14. Accordingly, during this period, the pressure in the
crossover passage is prevented from dropping below a predetermined
minimum pressure (typically 20, 30, or 40 bar absolute during full load
operation). Moreover, during a substantial portion of the engine cycle
(typically 80% of the entire engine cycle or greater), the XovrC valve 24
and XovrE valve 26 are both closed to maintain the mass of trapped gas in
the crossover passage 22 at a substantially constant level. As a result,
the pressure in the crossover passage 22 is maintained at a predetermined
minimum pressure during all four strokes of the engine's pressure/volume
cycle.

[0057] For purposes herein, the method of having the XovrC 24 and XovrE 26
valves open while the expansion piston 30 is descending from TDC and the
compression piston 20 is ascending toward TDC in order to simultaneously
transfer a substantially equal mass of gas into and out of the crossover
passage 22 is referred to herein as the Push-Pull method of gas transfer.
It is the Push-Pull method that enables the pressure in the crossover
passage 22 of the split-cycle engine 10 to be maintained at typically 20
bar or higher during all four strokes of the engine's cycle when the
engine is operating at full load.

[0058] As discussed earlier, the exhaust valve 34 is disposed in the
exhaust port 35 of the cylinder head 33 separate from the crossover
passage 22. The structural arrangement of the exhaust valve 34 not being
disposed in the crossover passage 22, and therefore the exhaust port 35
not sharing any common portion with the crossover passage 22, is
preferred in order to maintain the trapped mass of gas in the crossover
passage 22 during the exhaust stroke. Accordingly, large cyclic drops in
pressure are prevented which may force the pressure in the crossover
passage below the predetermined minimum pressure.

[0059] XovrE valve 26 opens shortly before the expansion piston 30 reaches
its top dead center position. At this time, the pressure ratio of the
pressure in crossover passage 22 to the pressure in expansion cylinder 14
is high, due to the fact that the minimum pressure in the crossover
passage is typically 20 bar absolute or higher and the pressure in the
expansion cylinder during the exhaust stroke is typically about one to
two bar absolute. In other words, when XovrE valve 26 opens, the pressure
in crossover passage 22 is substantially higher than the pressure in
expansion cylinder 14 (typically in the order of 20 to 1 or greater).
This high pressure ratio causes initial flow of the air and/or fuel
charge to flow into expansion cylinder 14 at high speeds. These high flow
speeds can reach the speed of sound, which is referred to as sonic flow.
This sonic flow is particularly advantageous to split-cycle engine 10
because it causes a rapid combustion event, which enables the split-cycle
engine 10 to maintain high combustion pressures even though ignition is
initiated while the expansion piston 30 is descending from its top dead
center position.

[0060] The split-cycle air-hybrid engine 10 also includes an air reservoir
(tank) 40, which is operatively connected to the crossover passage 22 by
an air reservoir (tank) valve 42. Embodiments with two or more crossover
passages 22 may include a tank valve 42 for each crossover passage 22,
which connect to a common air reservoir 40, or alternatively each
crossover passage 22 may operatively connect to separate air reservoirs
40.

[0061] The tank valve 42 is typically disposed in an air reservoir (tank)
port 44, which extends from crossover passage 22 to the air tank 40. The
air tank port 44 is divided into a first air reservoir (tank) port
section 46 and a second air reservoir (tank) port section 48. The first
air tank port section 46 connects the air tank valve 42 to the crossover
passage 22, and the second air tank port section 48 connects the air tank
valve 42 to the air tank 40. The volume of the first air tank port
section 46 includes the volume of all additional ports and recesses which
connect the tank valve 42 to the crossover passage 22 when the tank valve
42 is closed.

[0062] The tank valve 42 may be any suitable valve device or system. For
example, the tank valve 42 may be an active valve which is activated by
various valve actuation devices (e.g., pneumatic, hydraulic, cam,
electric or the like). Additionally, the tank valve 42 may comprise a
tank valve system with two or more valves actuated with two or more
actuation devices.

[0063] Air tank 40 is utilized to store energy in the form of compressed
air and to later use that compressed air to power the crankshaft 16, as
described in the aforementioned U.S. Pat. No. 7,353,786 to Scuderi et al.
This mechanical means for storing potential energy provides numerous
potential advantages over the current state of the art. For instance, the
split-cycle engine 10 can potentially provide many advantages in fuel
efficiency gains and NOx emissions reduction at relatively low
manufacturing and waste disposal costs in relation to other technologies
on the market, such as diesel engines and electric-hybrid systems.

[0064] By selectively controlling the opening and/or closing of the air
tank valve 42 and thereby controlling communication of the air tank 40
with the crossover passage 22, the split-cycle air-hybrid engine 10 is
operable in an Engine Firing (EF) mode, an Air Expander (AE) mode, an Air
Compressor (AC) mode, an Air Expander and Firing (AEF) mode, and a Firing
and Charging (FC) mode. The EF mode is a non-hybrid mode in which the
engine operates as described above without the use of the air tank 40.
The AC and FC modes are energy storage modes. The AC mode is an
air-hybrid operating mode in which compressed air is stored in the air
tank 40 without combustion occurring in the expansion cylinder 14 (i.e.,
no fuel expenditure), such as by utilizing the kinetic energy of a
vehicle including the engine 10 during braking.

[0065] The FC mode is an air-hybrid operating mode in which the
compression piston draws into the compression cylinder more air than is
needed to power the expansion stroke of the expansion cylinder during
combustion (i.e., the compressor draws in more air than is required to
power the expander). The excess compressed air, not needed for
combustion, is stored in the air tank 40, typically at less than full
engine load operating conditions (e.g., engine idle, vehicle cruising at
constant speed). The storage of compressed air in the FC mode has an
energy cost (penalty) in that additional negative work is required to be
performed by the compressor. Therefore, it is desirable to have a net
gain when the compressed air is used at a later time (i.e., to utilize
the compressed air in the expander to produce more positive work than
negative work required to store the excess air during the FC mode).

[0066] The AE and AEF modes are stored energy usage modes. The AE mode is
an air-hybrid operating mode in which compressed air stored in the air
tank 40 is used to drive the expansion piston 30 without combustion
occurring in the expansion cylinder 14 (i.e., no fuel expenditure). The
AEF mode is an air-hybrid operating mode in which compressed air stored
in the air tank 40 is utilized in the expansion cylinder 14 for
combustion.

[0067] In the FC mode, the compression piston 20 operates in its
compression mode, in that the compression piston draws in and compresses
inlet air for use in the expansion cylinder 14. The expansion piston 30
operates in its power mode, in that compressed air is admitted to the
expansion cylinder 14 with fuel, at the beginning of an expansion stroke,
which is ignited, burned and expanded on the same expansion stroke of the
expansion piston, transmitting power to the crankshaft 16, and the
combustion products are discharged on the exhaust stroke. The FC mode is
made possible because compression and expansion are split between the
compression cylinder 12 and the expansion cylinder 14. The expansion
cylinder 14 can be run at a load higher than the vehicle load. The excess
load is then absorbed by the compression cylinder 12 which compresses
more air than the expansion cylinder 14 requires to power the vehicle.
The excess (or extra) charge air is diverted to charging the air tank 40.

[0068] Significantly, while the engine 10 is operating in the FC mode, the
air tank valve 42 is kept closed until the XovrE valve 26 is
substantially closed during each single rotation of the crankshaft 16.
Accordingly, the expansion cylinder 14 is charged with compressed air
before the air tank 40 is charged with compressed air. Thus, during a
single rotation of the crankshaft 16, the expansion cylinder 14 and air
tank 40 are charged serially (i.e., one after the other, rather than at
the same time, which would be a parallel charging sequence). The
compressed air charge provided by the compression cylinder 12 during a
single rotation of the crankshaft 16 is thereby split between the
expansion cylinder 14 and the air tank 40.

[0069] Preferably, the air tank valve 42 at least remains closed from
within plus or minus 5 degrees CA of when the XovrC valve 24 opens (i.e.,
from when the XovrC valve is substantially open) to within plus or minus
5 degrees CA of when the XovrE valve 26 closes (i.e., to when the XovrE
valve is substantially closed). Thus, the air tank valve 42 is
substantially closed from a time (or a position in CA degrees) at which
the compressed air charge begins to enter the crossover passage 22
through the XovrC valve 24 to a time at which the compressed air charge
ceases to enter the expansion cylinder 14 through XovrE valve 26, thereby
preventing the air tank 40 from being charged before the expansion
cylinder. In an exemplary embodiment, the XovrC valve 24 may be opened at
a crankshaft position (valve timing) between approximately -23 and -10 CA
degrees ATDCe, and the XovrE valve 26 may be closed at a valve timing
between approximately 11 and 23 CA degrees ATDCe, as shown in FIGS. 6 and
9, respectively.

[0070] At all operating conditions of the engine 10, the air tank valve 42
is opened only after the XovrE valve 26 has closed. For example, the air
tank valve 42 may be opened at a position that is approximately 5 CA
degrees or greater after the XovrE valve has closed. Preferably, the air
tank valve 42 may be opened at a position that is in the range of 5-20 CA
degrees after the XovrE valve 26 has closed. More preferably, the air
tank valve 42 may be opened at a timing that is less than 10 degrees CA
after the XovrE valve has closed. The air tank valve 42 then may be held
open for a valve duration of 25 CA degrees or greater. Preferably, the
air tank valve 42 may be held open for a valve duration of 50 CA degrees
or greater. More preferably, the air tank valve 42 may be held open
within a range of 25 to 150 CA degrees, during which time the air tank 40
is charged with compressed air.

[0071] During one complete crankshaft revolution in the FC mode beginning
with the intake stroke of the compression piston 20 and ending with the
exhaust stroke of the expansion piston 30, the XovrC valve 24, the XovrE
valve 26, and the air tank valve 42 typically have the following sequence
of openings and closings. First, the XovrC valve 24 opens and then the
XovrE valve 26 opens. The crossover passage 22 is thereby pressurized
with compressed air from the compression cylinder 12, and the compressed
air is transferred to the expansion cylinder 14.

[0072] Typically, the XovrC valve 24 closes next, followed by the XovrE
valve 26 closing. However, under some engine operating conditions, the
XovrE valve 26 may close before the XovrC valve 24 closes. In either
case, an amount of excess compressed air is thereby trapped in the
crossover passage 22 between the closed XovrC and XovrE valves 24, 26.
The crossover passage 22 is over-pressurized such that the pressure in
the crossover passage is greater than the pressure in the air tank 40.
Next, the air tank valve 42 opens and then later closes, allowing the
excess compressed air in the crossover passage 22 to flow into the air
tank 40 due to the pressure differential between the crossover passage
and the air tank.

[0073] However, at certain engine operating conditions (e.g., engine
speed, engine load, air tank pressure, etc.), the air tank valve 42 may
open after the XovrE valve 26 has closed, but slightly before the XovrC
valve 24 has closed. In this case, the sequential order of valve openings
and closings is: XovrC valve 24 opens, XovrE valve 26 opens, XovrE valve
26 closes, air tank valve 42 opens, XovrC valve 24 closes, and air tank
valve 42 closes. Under this valve timing sequence, the XovrC valve 24 and
air tank valve 42 are open simultaneously for a short period of time,
providing fluid communication (i.e., an open fluid flow path) between
compression cylinder 12 and air tank 40.

[0074] Additionally, in the FC mode, the engine load may be controlled by
varying the timing of the XovrE valve closing to meter the needed amount
of air into the expansion cylinder required for combustion. As stated
above, in an exemplary embodiment, the XovrE valve 26 may be closed at a
valve timing between approximately 11 and 23 CA degrees ATDCe, as shown
in FIG. 9. Thus, the XovrE valve 26 only lets into the expansion cylinder
14 the amount of compressed charge air needed for the load required
(effectively by closing when the desired charge amount has entered the
expansion cylinder). The excess charge air remaining in the crossover
passage 22 is then stored in the air tank 40 as described above. The
amount of compressed air that is delivered to the air tank 40 during a
single rotation of the crankshaft 16 (and thus the air flowrate into the
air tank) may be controlled by varying the timing of the intake valve 18
closing, which effectively varies the total amount of charge air drawn
into the compression cylinder 12. In an exemplary embodiment, the intake
valve 18 is closed at a valve timing between approximately 103 and 140 CA
degrees ATDCe as shown in FIG. 2.

[0075] FIGS. 2 through 11 graphically illustrate an exemplary embodiment
of the FC mode of the split-cycle air-hybrid engine 10 described above
over a range of air tank pressures and air tank charging flowrates at an
engine speed of 2000 rpm and an engine load of 2 bar IMEP. In FIG. 2, the
intake valve 18 is closed at a timing in the range of 103.0 to 140.0 CA
degrees ATDCe. For example, at a tank pressure of 10 bar and an air tank
flowrate of 3 g/s, the intake valve 18 is closed at approximately 122 CA
degrees ATDCe. In FIG. 3, the intake valve 18 has a valve duration of
between 56.5 and 93.5 CA degrees. For example, at a tank pressure of 10
bar and an air tank flowrate of 3 g/s, the intake valve duration is
approximately 75 CA degrees.

[0076] In FIG. 4, the XovrC valve 24 has a valve duration of between 36.4
and 61.8 CA degrees. For example, at a tank pressure of 10 bar and an air
tank flowrate of 3 g/s, the XovrC valve duration is approximately 45 CA
degrees. In FIG. 5, the XovrE valve 26 has a valve duration of between
14.2 and 30.8 CA degrees. For example, at a tank pressure of 10 bar and
an air tank flowrate of 3 g/s, the XovrE valve duration is approximately
26 CA degrees.

[0077] FIGS. 6 and 7 depict the XovrC valve 24 opening and closing
timings, respectively. The XovrC valve 24 opens at a timing in the range
of -23.20 to -9.79 CA degrees ATDCe and closes at a timing in the range
of 24.6 to 38.6 CA degrees ATDCe. For example, at a tank pressure of 10
bar and an air tank flowrate of 3 g/s, the XovrC valve 24 opens at
approximately -17.5 CA degrees ATDCe and closes at approximately 28 CA
degrees ATDCe.

[0078] FIGS. 8 and 9 depict the XovrE valve 26 opening and closing
timings, respectively. The XovrE valve 26 opens at a timing in the range
of -1.62 to 14.00 CA degrees ATDCe and closes at a timing in the range of
11.40 to 23.20 CA degrees ATDCe. For example, at a tank pressure of 10
bar and an air tank flowrate of 3 g/s, the XovrE valve 26 opens at
approximately -7.3 CA degrees ATDCe and closes at approximately 19 CA
degrees ATDCe.

[0079] FIGS. 10 and 11 depict the air tank valve 42 opening and closing
timings, respectively. The air tank valve 42 opens at a timing in the
range of 21.4 to 33.2 CA degrees ATDCe and closes at a timing in the
range of 131.4 to 143.2 CA degrees ATDCe. For example, at a tank pressure
of 10 bar and an air tank flowrate of 3 g/s, the air tank valve 42 opens
at approximately 29 CA degrees ATDCe and closes at approximately 139 CA
degrees ATDCe.

[0080] As can be seen from FIGS. 9-11, over the range of air tank
pressures and air tank charging flowrates, in this exemplary embodiment
the air tank valve 42 opens 10 CA degrees after the XovrE valve 26
closes, and the air tank valve closes 110 CA degrees after it opens
(i.e., the air tank valve duration is substantially fixed at 110 CA
degrees).

[0081] The above exemplary embodiment has illustrated a valve timing
sequence for the FC mode at a single engine speed and load (i.e., 2000
rpm at 2 bar IMEP). However, one skilled in the art would recognize that
the FC mode may operate over the entire speed and load range of the
engine 10. That is, the FC mode may operate from no-load to full-load and
from idle speed to rated (full) speed of engine 10.

[0082]FIG. 12 graphically illustrates the fuel (i.e., energy) penalty for
compressing excess air in the compression cylinder 12 (for subsequently
charging the air tank 40) in the FC mode at an exemplary engine speed of
2000 rpm and engine load of 2 bar IMEP. The horizontal line at the bottom
of the graph (air tank charging rate of 0 g/s) represents the fuel
flowrate (in kg/hr) when the air tank 40 is not being charged
(essentially the EF (or NF) mode of the engine 10). This is the zero fuel
penalty baseline from which the fuel penalty in the FC mode is
calculated. The three lines above the horizontal baseline represent fuel
expenditures in the FC mode at air tank charging rates of 1 g/s, 2 g/s,
and 3 g/s. The fuel expenditures in the FC mode are, of course, greater
than the fuel expenditure in the EF mode. The fuel penalty in the FC mode
is calculated by subtracting the baseline fuel expenditure from the fuel
expenditure at a specific air tank pressure and air tank charging rate.
For example, at an air tank pressure of 5 bar and an air tank charging
rate of 2 g/s, the fuel penalty (extra energy spent to charge the air
tank) is approximately 0.09 kg/hr (1.11 kg/hr at 5 bar and 2 g/s minus
the baseline expenditure of 1.02 kg/hr). As another example, at an air
tank pressure of 10 bar and an air tank charging rate of 3 g/s, the fuel
penalty is approximately 0.35 kg/hr (1.37 kg/hr minus 1.02 kg/hr).

[0083] Although the invention has been described by reference to a
specific embodiment, it should be understood that numerous changes may be
made within the spirit and scope of the inventive concepts described.
Accordingly, it is intended that the invention not be limited to the
described embodiment, but that it have the full scope defined by the
language of the following claims.